Carburateurs Dell'Orto - Guide technique

LA GUZZITHÈQUE 15/21 19/03/04
3.5 PROGRESSION SYSTEM
By progression we mean the transition period between mixture delivery from the idle circuit and the beginning of mixture
delivery from the main jet circuit.
On first opening the throttle, the air drawn into the engine increases and therefore, in order to have an inflammable mixture
still, the fuel supply must also be increased.
fig. 20
3.6 FULL-THROTTLE OPERATION
As previously noted, the idle hole(3) shown in figure 20, only delivers sufficient fuel
for engine idle operation and the main circuit still does not deliver any fuel because
of insufficient vacuum up stream of the throttle. The progression hole (2) is therefore
necessary to deliver the fuel required during this transition period. The progression
hole draws fuel from the idle circuit (4) and is positioned immediately upstream of
the closing edge of the throttle slide (1) for the promptest response to fuel demand
when the airflow suddenly increases. It is interesting to note that the progression hole
serves a dual purpose: When the engine is idling, air from the main barrel passes into
the progression hole and weakens the mixture flowing through the idle circuit; When
the throttle is opened slightly, the idle circuit mixture flows into the main barrel
through the progression hole.
The progression hole therefore first feeds air in one direction and then feeds mixture
in the opposite direction.
Following the progression phase, on further opening of the throttle, the full-throttle circuit begins to operate. By opening the
throttle valve beyond progression, a partial vacuum is created in the mixture chamber, due to the speed of the air being drawn
through to the engine, and this vacuum is sufficient to cause fuel to be sucked out of the atomiser nozzle.
fig. 21
In this situation (figure 21), fuel metered by the main jet (5) and further regulated by the
atomiser outlet (3) (the atomiser outlet area varies according to the position of the taperedneedle
moving up and down through it) is mixed with air from channel (4) and air from the
main barrel (2).
The amount of fuel which comes out in the first quarter of the throttle slide movement is
determined by the throttle slide cutaway, by the size of the atomiser and by the diameter of
the cylindrical part of the tapered-needle at the opening.
From here up to three-quarter throttle, it is deter mined by the atomiser-needlejet size and
by the diameter of the tapered-needle at the opening.
From three-quarter throttle to full throttle the amount of fuel depends solely on the size of
the main jet.
Therefore you should change the following parts to vary the full throttle circuit delivery:
• the throttle slide cutaway
• the tapered needle
• the atomiser-needlejet size and type
• the main jet
There are two different full-throttle systems; one is used on two-stroke engines and the
other on four-strokes, although some special applications do not conform to this.
LA GUZZITHÈQUE 15/21 Dellorto Tuning Guide